Most conventional commercial vehicle trailers are a development of the original two axle horse-drawn cart, and are equipped with a front axle assembly which pivots about a vertical axis and a drawbar which is connected to the front axle assembly so that the rear of the drawbar pivots about a horizontal transverse axis. The full weight of the trailer is accordingly supported by its front and rear axles and, if the trailer is braked, the only significant hitch loads at the rear of the towing vehicle are those required to pull the trailer. However, the inherent weakness of this design of trailer is that it has two articulation points: one at the hitch point at the rear of the towing vehicle, and the second at the vertical pivot axis of the front axle assembly. Various problems arise from the presence of two articulation points. Firstly, the drawbar tends to jackknife when the vehicle combination is operated in reverse, so that it is impractical to manoeuvre such a trailer in reverse except for short distances in a straight line. Secondly, the drawbar tends to jackknife under emergency braking. Thirdly the trailer suffers from the phenomenon known as rearward amplification. That is to say, the trailer swings more than the towing vehicle during high speed steering manoeuvres. This is a particular problem when several trailers are connected together.
Various designs have been proposed to overcome the fundamental disadvantage of the conventional trailer, which is often referred to as an "A" dolly trailer. However, practical factors such as cost and incompatibility with existing equipment have limited their widespread use.
The most common alternative designs are referred to as a "B" train and a "C" dolly (also referred to as a "B" dolly). In the "B" train, the rear of the frame of the lead trailer is extended to accommodate a fifth wheel for coupling to the following semi-trailer. This eliminates one articulation point, with an improvement in stability and manoeuvrability. However, the disadvantages compared with an "A" dolly trailer include a degradation of the off-tracking performance, increased frame stressing, cost, and incompatibility with conventional "A" dolly trailers. Further problems follow from the use of two or three axles in the area of the fifth wheel, which results in increased tire wear, and in an increase in road load concentration which would be unacceptable in some jurisdictions. Only some of these problems are overcome with the use of a "C" dolly, which is connected to the lead trailer with two pintle hooks but which permits articulation of the rearmost axle. Finally, for many practical reasons, the "B" train and its derivatives are not suitable for connecting a truck and conventional "A" dolly trailer.
In summary, the conventional "A" dolly trailer design benefits from having the entire weight of the trailer supported by its own axles. However, the presence of two articulation points adversely affects both its dynamic stability and its ability to operate in reverse. In the "B" train and "C" dolly trailer designs, the advantages of having only one articulation point are offset by the disadvantages associated with having part of the trailer weight directly supported by the towing vehicle. This results in increased frame stressing, large road load concentration, and increased tire wear.